It works well for the actual purpose. But I'd be happy about a more accessible format for development. Currently whenever I need accurate reading/writing capabilities for PDF in a Rust project, at some point I fear that I might have to dedicate my life to PDF. And then I just give up.
PDF writing isn't too bad IMO, since you don't need to understand the whole spec. I've written a PDF writer for maps from scratch and it was fairly easy and not too much code.
PDF reading though... Yeah I'm happy to leave that to people with more time and use their libraries.
A modern format would be nice, but I don't think it would be anywhere near nice enough to give up how universal PDF is.